Television transmission signals typically account for a vertical blanking interval (VBI). The VBI refers to the time required by the electron gun in a conventional television set (or similar visual display) to reset itself to the top of the television screen from the bottom of the screen. Conventional television sets and many other visual displays generate their display by using an electron gun(s) to illuminate each line on the television screen. The electron gun illuminates a single line at a time and typically starts at the top of the screen, illuminates every other line as it travels down the screen, then resets itself to the top of the screen once it reaches the bottom. During this reset from the bottom of the screen to the top of the screen, no lines of the screen are being illuminated. This period of reset is referred to as the VBI.
During the VBI, no video information is transmitted by a broadcasting device because the electron gun is not illuminating lines at that time. Thus, control information and/or other data can be transmitted during the VBI rather than video information without disrupting the video and/or audio transmission. Typically, a portion of the VBI is used by television broadcasters for control information necessary for displaying the video information. This control information includes, for example, video synchronization signals. In addition, a portion of the VBI is typically reserved for broadcasting the text for closed captioning for the hearing impaired. However, unused space remains in the VBI which can be used by a broadcaster to broadcast any of a wide variety of data, typically referred to as teletext data, of the broadcaster's choosing.
Digitizing hardware for capturing teletext data typically samples each teletext scan line with a particular sampling rate, which is typically greater than the teletext's data bit frequency. The sampling rate used by different hardware capture devices can vary. However, the decoder needs to know the sampling rate used by the hardware capture device it is receiving data from in order to properly decode the data. Therefore, given the variety of commercially available capture devices, it would be beneficial to have a single decoder which would automatically support different sampling rates.
One solution to this problem is to provide different customized software code for each of the possible sampling rates. However, providing such customized code for each possibility is impractical because the number of possibilities is too large. Thus, it would be beneficial to provide a mechanism to automatically adjust to different sampling rates without requiring specific customized code for each possibility.
As will be described in more detail below, the present invention provides a method and apparatus for supporting variable sampling rates when decoding vertical blanking interval data to achieve these and other desired results which will be apparent to those skilled in the art from the description that follows.